Moreover, without the vibration condition, undershoot time is very short. Undershoot time depends on the diffusion velocity of air in the gas diffusion layer when the gas is enough in the flow channel. On the contrary, when the gas is not enough in the flow channel, the undershoot time depends on the gas transport in the flow channel. In the experiment, the velocity remains unchanged before and after loading. So, the gas is enough in the flow channel, the time to penetrate the GDL for gas is far less than 1 s. This is consist with time computational formula of gas penetrate the GDL